The term "air conditioning" has been broadly defined to mean the maintenance of certain aspects of the environment within a defined space. Environmental Conditions, such as air temperature and motion, moisture level, and concentration of various pollutants, are generally encompassed by the term.
Comfort air conditioning refers to control of spaces inhabited by people to promote their comfort, health and productivity. Spaces in which air is conditioned for comfort include residences, offices, institutions, sports arenas, hotels, factory work areas, and so on.
With recent trends being directed to maintaining quality levels of clean air as today's society has become more health and environmentally aware, a greater emphasis is being placed on the purification components of air conditioning systems. At its simplest level, air pollution control suggests a background knowledge concerning desirable criteria for clean air, the ability to relate air quality to levels of emissions, the development of emission limits or other control standards, the means to measure such emissions and air quality, and the availability of practical techniques to reduce air pollutants. Therefore, although increasing attention has been directed to process alterations to reduce air-pollutants in general, great reliance is still placed upon physical removal processes.
A complete air conditioning system is capable of adding and removing heat and moisture. Moisture is typically added to provide an environment comfortable for human occupancy. In addition, such systems can filter dust and odorants from the space or spaces it serves. Generally, cold weather air conditioning systems are designed to heat, humidify and filter for cold weather comfort while warm weather air conditioning systems cool, dehumidify and filter. Typically, design conditions are such that both cold and warm weather air conditioning can be maintained by multiple independent subsystems together by a single control.
To control humidity and air purity (and in most systems for controlling air temperature), a portion of the air in the space is withdrawn, processed, and returned to the space to mix with the remaining air. Such air-handling units generally contain a filter, a cooling coil, a heating coil, and a fan in a suitable casing.
Although the filter removes dust and other pollutants from both return and outside air, the gaseous pollutant removal efficiencies and performance of such filters are still considerably less than other low cost air purification alternatives (e.g., ventilation) because of the very low concentrations of pollutants found in areas of human occupancy. For example, low concentrations of pollutants such as formaldehyde, sulfur dioxide, and nitrogen dioxide are generally found in levels less than 100 ppb (parts per billion). As such, current filter systems are not cost effective for active indoor air quality control, i.e. human habitats, office buildings, etc. In these applications, for example, the air pollution removal (APR) devices performance is limited, e.g., pollutant removal efficiency, E.sub.C &lt;50% and reagent utilization, (the amount of reagent used of total reagent available , U.sub.R &lt;10%. Therefore, a need exists to improve the performance of such filters while maintaining acceptable capital and operating costs. Only then will APR devices become an integral part of air conditioning systems and an economically attractive alternative in environments harboring low levels of gaseous pollutants.
While the normal approach for the filtering of air passing through an air conditioning system involves the filtering of the entire air flow volume, such an arrangement may not be practical for the process of filtering gaseous pollutants. One of the reasons is that, in order to obtain the degree of filtering that is necessary, the density of the filter has to be such that a relatively high pressure drop occurs across the filter. As an alternative, the cross sectional area of the filter may be increased such that the pressure drop is brought down to an acceptable level. However, neither the high pressure drop nor the relatively large cross sectional area is considered practical in a conventional residential system. In addition, the desire for a relatively low velocity of air flow in order to increase the dwell time in a gaseous pollutant filter, makes it difficult to perform the filtering function at a point in the primary air flow stream. For example, activated carbon filters have been installed in the primary airflow duct of air circulation systems. But, because of the problems mentioned above, such a system necessarily involved either in a relatively high pressure drop that may necessitate the use of an auxilary air mover, or the use of a rather porous and relatively inefficient filter structure airstream. In either case, however, the velocity of the airstream is relatively high and the dwell time within the filter is therefore low. It is therefore difficult to obtain the kind of performance efficiency that is desirable for a chemical filter.
In the humidification of air being supplied to a space, the above considerations are also applicable. That is, the need for relatively high pressure drops and lower flow velocities has prompted the use of a bypass arrangement for humidifying a portion of the air being returned from the space to which the conditioned air is provided. In that case, however, moisture is being added to the air rather than contaminants being removed as in the case of a filtering process. Accordingly, for that air being bypassed, moisture can be added to the air to an extent that the air is "over humidified", and that "over humidified" air can then be mixed with the air flowing in from the return air duct in order to obtain the desired level of humidity in the mixture which is then delivered to the space. This is not true in the case of a filtering function wherein, rather than adding moisture to the air being conditioned, gaseous pollutants are removed from the air. Further, the air cannot be "over filtered", such that when mixed with the return air the resulting mixture is then free of gaseous polutants to the degree desired.